In the Media Pool, click category 
Timelines and double-click the required timeline.
Click button Display to disply the corresponding timeline.
If the tracks in your project suddenly disappear you may have closed the Timeline window accidentally. This is how you can open it up again:
In the Media Pool, click category Timelines and double-click the required timeline.
Click button Display to disply the corresponding timeline.
Following this the tracks should be visible again.